1. Field of the Invention
This invention relates to a balancer structure for an engine in which two balancer shafts are provided for one crankshaft.
2. Description of Related Art
Of engines with a large displacement, single-cylinder or V-type two-cylinder engines, for example, are in use in a lower engine speed range compared with multi-cylinder engines such as four-cylinder engines. Therefore, a vibration due to an inertia force becomes dominant. In some engines of this type, for the purpose of canceling the inertia force, a phase-pin crank has been adopted, which has a phase difference in the position of the crank pins or a two-shaft primary balancer has been adopted, which has two balancer shafts. By the way, in a one-shaft primary balancer with one shaft, the moment of the inertia force cannot be canceled and a vibration reducing effect is small especially in engines with a large displacement.
However, when the foregoing phase-pin crank is adopted, since pins with different phases are connected, a web is required between pins and the distance between crank journals as supporting points is necessarily increased compared with a coaxial pin crank, which is disadvantageous to the strength or the rigidity of the crankshaft. In engines with a displacement, for example, over 1000 cc in particular, a large web thickness and thus a large minimum sectional area are required to secure the strength sufficient enough to withstand increased torque variations, so that a longer crankshaft is needed and the width of the crankcase is enlarged that much.
In contrast, when the foregoing two-shaft primary balancer is adopted, although an increase in width of the crankcase can be prevented, the two balancer shafts should be disposed in front of and behind the crankshaft, so that the length of the crankcase is increased.
In view of the foregoing, an advantage of the invention is to provide a balancer structure for an engine capable of suppressing an increase in the longitudinal length of a crankcase when a two-shaft primary balancer is adopted.